The present invention relates to a bush with a spherical sliding surface including a socket and a ball-integral shaft with shaft portions on both sides of a ball portion, which is suitable, but not exclusively, for use in suspension systems of automobiles.
As a bush for use in a rear suspension of an automobile, for example, as a bush for use in a connecting portion of a torque rod and a suspension arm, a universal bush with a socket and a shaft which extends through both sides of the socket is used. A bush which is available for such a use is disclosed in the journal ATZ 84(1982) 7/8, page 355. The bush includes a ball-integral shaft with a ball portion and shaft portions integrally provided on both sides of the ball portion and has a spherical sliding surface between the ball portion of the ball-integral shaft and a bearing body housed in a socket.
Such a bush has a small sliding surface for bearing a load, because the spaces occupied by the shaft portions of the ball-integral shafts are not available as a space for providing a load bearing surface. Therefore, to increase load bearing abilities of the bush, hard thermo-plastic resins having high load bearing characteristics, such as polyacetals, have been used for the material of the bearing body. However, bearing bodies constructed of polyacetals can scarcely absorb manufacturing tolerances of parts of the bush because of their very limited resiliency. Further, it is difficult to keep the rotational torque of the bush at appropriate levels and to obtain a good driving feeling. More specifically, if the bearing body is interference-fitted into the socket with too large of an interference to make additive effects of manufacturing tolerances small, the rotational torque of the bush will be increased to an unacceptable level. Conversely, if the bearing body is loosely fitted into the socket to make the rotation of the shaft smooth, additive effects of manufacturing tolerances of parts of the bush will be increased to a unacceptable level, resulting in decreased driving feeling and bush durability. Therefore, in bushes using hard resins, such as polyacetals, as the material of the bearing body, it is very difficult to satisfy both characteristics of smooth rotation and high durability, though the bush load bearing characteristics are increased.
Apart from a bush with shafts on both sides, a ball and socket joint including a ball-pin with a single shank on one side of a spherical head is disclosed in Japanese Utility Model Publication No. SHO 55-179222 and Japanese Utility Model Publication No. SHO 58-172137. In these two ball and socket joints, the bearing body is divided into two members which are separated from each other. One member is pushed against the spherical head of the ball-pin by a resilient means, such as a metallic coil spring, a rubber means or a resilient rib means, so that the pre-load imposed on the bearing bodies can be adjusted. The resilient means is interposed between the spherical head of the ball-pin and the bottom wall of the socket. Such an arrangement can be adopted in this ball and socket joint because the space on the side of the spherical head where the shank does not extend can be used as a space to provide the resilient means. In the pre-loading mechanism, the rotational torque of the bush is adjusted substantially only by deformation of the resilient means, because deformation of the bearing body itself is not used for the adjustment of the rotational torque of the bush.
When attempting to apply the pre-load adjusting mechanisms disclosed in a ball and socket joint which has a shaft on only one side of the joint to the bush having shaft portions on both sides of the bush, there are a lot of difficulties as mentioned below.
a. A bush differs to a great extent from a ball and socket joint in structure. In a bush, the space occupied by the shaft portions can not be used as a space for providing a resilient means for giving a pre-load to the bearing body, while in a ball and socket joint a comparatively large space on the side where the shank does not extend can be used as a space for providing the resilient means. In a ball and socket joint, a large-sized resilient means which inevitably becomes of a large size to obtain sufficient axial force can be housed in the socket using the above-mentioned large space, but there is no such a large space in a bush. Therefore, a resilient means as disclosed in the above-mentioned Japanese Utility Model Publications, which are constructed of a metallic coil spring and a rubber means can not be used in a bush.
b. Torque adjustment of a bush with a single bearing body is performed via the deformation of the bearing body itself and therefore the torque of the sliding surface of the bush is greatly affected by the deformation of the bearing body, while in a ball and socket joint the torque of the sliding surface is adjusted only by the deformation of the resilient means, without a substantial deformation of the bearing body. Due to the difference in the torque adjustment mechanism, the same torque adjustment mechanism as that of a ball and socket joint can not be applied to a bush.
Due to the above-mentioned reasons, conventional bushes have been used without a special preload-adjustment means resulting in the above-mentioned problems.